Researchers demonstrate colour-tunable graphene-based LED

Jan 18, 2018: A team of researchers from the Université de Strasbourg (France) and the CNR-Nanoscience Institute (Modena, Italy) have demonstrated that due to highly tunable band gaps, graphene nanoribbons (GNRs) with atomically precise edges are emerging as mechanically and chemically robust candidates for nanoscale light emitting devices of modulable emission color. They have revealed their research in a paper entitled “Bright Electroluminescence from Single Graphene Nanoribbon Junctions” published in ACS’ Nano Letters.

Graphene nanoribbons (GNRs), with atomically precise edges, were already known for their highly tunable band gaps. When cut into thin ribbons, just a few atoms wide, graphene obtains a sizable optical band gap, which is not possible with full graphene sheets that do not have any optical band gap.

The electroluminescence of individual GNRs suspended between the tip of a scanning tunneling microscope (STM) and a Au(111) substrate, constituting thus a realistic optoelectronic circuit. Emission spectra of such GNR junctions reveal a bright and narrow band emission of red light, whose energy can be tuned with the bias voltage applied to the junction, but always lying below the gap of infinite GNRs. Comparison with ab initio calculations indicates that the emission involves electronic states localized at the GNR termini. Our results shed light on unpredicted optical transitions in GNRs and provide a promising route for the realization of bright, robust, and controllable graphene-based light-emitting devices.

The European researchers have found a bright and narrow band emission of red light from individual graphene nanoribbons, only 7-atom-wide, at a high intensity comparable to bright light-emitting devices made from carbon nanotubes. Optical emission was up to 10 million photons per second, about 100 times more intense than the emission measured for previous single-molecular optoelectronic devices, the researchers claim.

The researchers have also found that the energy shift of the main peak changes as a function of the voltage, which provides a way to tune the colour of the light. To create an electronic-like circuit, they suspended individual GNRs between the tip of a scanning tunnelling microscope (STM) and a gold substrate Au(111), forming an optoelectronic circuit. They found that the bright emission involves electronic states localized at the GNR termini.